"I like to think of the building as somebody sunbathing on a lilo in Marbella," says Ted Cullinan with a mischievous grin. He is sitting in the sunken courtyard of his latest project, a new Maggie's Centre for cancer care, surrounded by a tarmac landscape of car parks and sheds, in the grounds of Newcastle's Freeman hospital. It requires something of a leap of imagination to summon the Costa del Sol.

But Cullinan persists: "Look, it has its tummy exposed to capture the warmth of the sun," he says, pointing his walking stick towards the south-facing walls of glass. "While its shoulders keep cool in the sea" – the Mediterranean here being a mound of earth that shelters the building's northern face. Where we are sitting, he says, feels five degrees warmer than it would do, because of the protective earthen berm that blocks out the breeze, and the radiant panels of rusted corten steel that clad the facade.

For the last five decades, the 81-year-old architect has used similar metaphors to demonstrate how easily the natural environment can be harnessed to make spaces that are simply comfortable to be in, and use a little less energy in the process. Designing with the triad of "long-life, loose fit, low energy" since the 1960s, his buildings are worn by their users like a favourite cosy cardigan or pair of jeans, improving with age. For Cullinan, the idea of "sustainable design" – since commodified by others in a high-tech language of grills and flaps, gizmos and louvers – is basic common sense.

"People say Newcastle is always windy and freezing, that it has a horrible climate" he says. "But it doesn't – it's just the way we use it. Buildings can use their climate so easily, that it's such a shame not to do it."

The origins of his design for the Maggie's Centre are now framed on the wall of the reception, where watercolour sketches show a pagan sunburst, with a curious crimson face, beaming health-giving rays down over the building. A circular roof arches upwards to catch the sunbeams, forming a Pringle-shaped cap of photovoltaic cells, while rooms extend below in an L-shaped plan, like arms outstretched to embrace the sun. The building is a conscious celebration of its response to the elements.

This is the 16th Maggie's Centre, a programme begun by architectural theorist Charles Jencks and his wife Maggie Keswick, shortly before she died of cancer in 1995, to provide light and airy "homes from home" for cancer patients to receive a less clinical kind of care. Cullinan joins an all-star cast, with buildings by Frank Gehry and Zaha Hadid, Richard Rogers and Rem Koolhaas, each project a distillation of their architectural manifestos.

"Each one is like an experiment in a petri dish," says Jencks, who continues to expand his laboratory, with forthcoming centres by Wilkinson Eyre, Steven Holl and Norman Foster among others. "They are all mini icons." So, after Gehry's crumpled tinfoil roof and Hadid's faceted concrete bunker, what kind of icon is Cullinan's?

"In the 1980s, Ted was the spearhead of a movement called 'romantic pragmatism'," says Jencks, who is no stranger to coining architectural taxonomies, as the godfather of postmodernism. "It isn't just green, warm and cuddly architecture. It's a very British, practical sensibility, mixed with platonic geometric forms and sometimes cosmic motifs" – as the mysterious celestial pods of Cullinan's Cambridge mathematics faculty demonstrate.

The staircase is lined with cubbyholes for reading. Photograph: Paul Raftery

In Newcastle, the overriding geometry is a simple grid of concrete columns and beams, forming a nine-bay lattice across the site. Into this concrete shell – left exposed to absorb heat during the day – timber-bordered walls and partitions are slotted like pieces of movable furniture, giving the interior a Japanese sense of layered space.

The grid defines generous square rooms at either end of each wing – one for exercise, one for cooking and dining – with smaller private therapy rooms tucked in along the edges. A double-height living room and library stands at the centre of the plan, as the fulcrum from which the wings extend, with a staircase rising up to the timber ribs of the roof, which arcs up like the bulging hull of a ship.

Every surface has been turned into a potential place to cosy up with a cuppa and a book, with window seats and cubby holes lining the library shelves up the stairs, all upholstered in cheery fabrics.

"It's basically a very large house," says Cullinan. "A building where you relax, read, cook, take exercise: all of the elements of 'uomo universale'."

"Uomo", he says, because he was particularly keen to attract men to use what are often seen as "overly genderised" buildings for women. The centre is launching "Maggie's Men's Mondays", while outdoor exercise equipment on the rooftop garden is also part of the attraction, where lawns are framed by beech hedges – which will hopefully soon hide the rather clunky metal balustrade. Cullinan is also keen to introduce a croquet set on the roof: "It's a very good game for people who aren't feeling so confident," he says, swinging an imaginary mallet between his legs. "Because it's so vicious!"

Croquet might not have fared so well on the rooftop of the previous design for the site, produced by Foreign Office Architects in 2009, which was conceived as a continuous landscape rising up out of the ground in the manner of a Teletubby house. The architects withdrew when the site was changed – and it is unlikely that their undulating vision could have been built for the £1.5m construction budget of this project.

Cullinan's Maggie's Centre may be one of the less striking of Jencks' architectural collection from the outside, looking fortress-like with its rusty steel walls behind defensive mounds of earth and high railings marching around the rooftop. But within, it is undoubtedly one of the most humane and comfortable of the lot, and once the planting matures to soften some of the harder edges, his particular brand of "romantic pragmatism" will shine through.